Light-emitting diode (LED) is a solid state semiconductor device and generally comprises a p-type semiconductor layer, an n-type semiconductor layer, and an active layer formed between the p-type semiconductor layer and the n-type semiconductor layer for emitting light under the principle of transforming electrical energy to optical energy by injecting electrons and holes through the n-type semiconductor layer and the p-type semiconductor layer respectively to the active layer to perform radiative combination and emit light.